package coo1;

import java.util.Scanner;
import java.util.Stack;
import java.util.regex.Pattern;

public class Houzhui {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        System.out.println("输入表达式，以空格进行分割。例：9 3 1 - 3 * + 10 2 / + ");
        String str = sc.nextLine();// 空格分割，按行读取
        count(str);
    }

    /*
     * 用于判断字符时数字还是运算符
     */
    public static boolean isNumeric(String string) {
        Pattern pattern = Pattern.compile("[0-9]*");
        return pattern.matcher(string).matches();
    }

    /*
     * 实现计算操作
     */
    public static void count(String str) {
        float op1 = 0;// 记录出栈数字
        float op2 = 0;// 记录出栈数字
        float result = 0;// 记录运算结果
        Stack<String> stack = new Stack();// 创建栈对象

        String[] arr = str.split(" ");// 对传入的字符串通过空格进行分割，并传入数组

        for (int i = 0; i < arr.length; i++) {
            // 对字符进行判断，如果为数字，则入栈；如果为运算符，则进行运算。
            if (isNumeric(arr[i])) {
                stack.push(arr[i]);
            }  else if (arr[i] == "+" || arr[i] == "-" || arr[i] == "*"
                    || arr[i] == "/") {
                op1 = Float.parseFloat((String) stack.pop());
                op2 = Float.parseFloat((String) stack.pop());
                // 根据运算符，选择合适的运算
                switch (arr[i]) {
                    case "+":
                        result = add(op2, op1);
                        stack.push(result + "");
                        break;
                    case "-":
                        result = substaction(op2, op1);
                        stack.push(result + "");
                        break;
                    case "*":
                        result = multiply(op2, op1);
                        stack.push(result + "");
                        break;
                    case "/":
                        result = divide(op2, op1);
                        stack.push(result + "");
                        break;

                    default:
                        break;
                }
            } else {
                System.out.println("输入有误，请重新输入！");
            }
        }
        System.out.println("后缀表达式计算结果为：" + result);
    }

    public static float multiply(float a, float b) {
        return a * b;
    }

    public static float divide(float a, float b) {
        return a / b;
    }

    public static float add(float a, float b) {
        return a + b;
    }

    public static float substaction(float a, float b) {
        return a - b;
    }

}
